The present invention relates to an apparatus and method for removing the curl from a running web of material being delivered from a storage roll. More particularly, the apparatus and method of the present invention are particularly suitable for decurling a paper or paperboard web, but is also applicable to webs of plastics and other materials.
It is well known in the art that web material which has been wound tightly on a roll takes a set such that the web or sheets cut from the web exhibit a curl which is manifested by a concave face on the inside face of the web with respect to its orientation on the roll. Furthermore, the curl becomes more pronounced nearer the center of the roll. Because the curl in a web or in sheets cut from the web generally interferes with and is detrimental to downstream conversion processes, much attention has been devoted in the industry to methods and apparatus for removing curl. Such efforts have focused primarily on removing the curl immediately downstream from the supply roll and before the web enters the conversion process.
Most decurling apparatus operates by causing the web to be back wrapped around a decurler bar or decurler roll in the opposite direction from which the web was wound on the supply roll. Typically, the decurler bar or decurler roll has a relatively small diameter so that a small radius back wrap is imparted to the running web to remove the curl. In one common type of decurler, the running web traveling between a pair of transfer rolls with the inside face of the web in contact with the rolls is contacted on its opposite face with a small diameter decurler roll that deflects the web out of its path of travel and causes it to partially wrap around the circumference of the decurler roll. It is also known to place the decurler roll in close proximity to one of the transfer rolls but, because the back wrap or back bending of the web around the smaller diameter decurler roll is much more severe, the set caused by the original curl may be reversed and the web flattened.
However, back bending a web around a small diameter roll often results in damage to the surface of the sheet. This damage, commonly referred to as checking or cracking, appears as permanent wrinkles or creases throughout the surface of the web on the face that contacts the small diameter roll. Checking or cracking is the result of compressive failure of the web face and can adversely affect the surface of a paper web, the coating on a web, and/or the bond between the coating and the paper. Other web materials may be similarly adversely affected.
The subject invention arose out of an investigation into the causes of cracking and checking induced by back bending or back wrapping the web around a decurler roll. It has been discovered that tensioning the web prior to decurling eliminates checking and cracking. It is believed that, by raising the overall web tension prior to back wrapping the web around the small diameter decurler roll, the inside face of the web in contact with the decurler roll can absorb the compressive forces of bending without failure resulting in checking. Simultaneously, the outside of the pretensioned web is stretched further due to the tensile forces of bending. The stretching of the outside web face results in a yielding of the paper fibers which removes the curl or set. It is believed that the yielding of the outside face moves the neutral surface of the sheet back toward the center of the web, removing residual stresses and creating a flat sheet.
Another important aspect of the method and apparatus of the present invention is to create the increased tension in the web for decurling without affecting web tension upstream from the web supply roll and downstream in the converting process operating at a desired line tension. If desired, however, the apparatus of the present invention may be operated to purposely change or adjust line tension downstream of the decurling apparatus. In other words, a line tension may be provided that is higher or lower than tension of the incoming web from the web supply.
In accordance with the method of the present invention, a running web delivered from a supply roll to a downstream conversion process operating at line web tension is decurled with a method comprising the steps of (1) creating a zone of increased web tension greater than line tension downstream of the supply roll and upstream of the process, (2) positioning a rotatable decurler roll in contact with one face of the web in the zone of increased web tension, and (3) adjustably positioning the decurler roll to selectively deflect the web from a normal path of travel through the zone and to vary the amount of circumferential wrap of the web on the decurler roll. In accordance with the preferred method, the zone of increased tension is created by (1) passing the web around a brake roll at an upstream end of the zone, and (2) passing the web around a pull roll at a downstream end of the zone.
In accordance with the preferred embodiment, the method also includes the steps of (1) positioning a rotatable pivot roll in contact with the opposite face of the web immediately adjacent the decurler roll, and (2) adjusting the position of the decurler roll to simultaneously vary the amount of circumferential wrap of the web on the decurler roll and the pivot roll. The adjusting step preferably comprises moving the decurler roll in an orbital path around the circumference of the pivot roll.
The method also preferably includes the steps of (1) passing the web around an infeed idler roll upstream of the brake roll, and (2) passing the web around an outfeed idler roll downstream of the pull roll. The method also preferably includes the steps of (1) providing separate drives for the brake roll and the pull roll, and (2) adjusting the drives to selectively vary web tension in the zone.
The preferred apparatus of the present invention comprises an upstream brake roll that receives and rotatably engages the web delivered from the supply roll, a downstream pull roll that receives and rotatably engages the web received from the brake roll, the brake roll and pull roll operative to create in the web therebetween a zone of web tension greater than line web tension, a decurler roll positioned in the web tension zone rotatably engaging the outer face of the web with respect to web orientation on the supply roll, and a decurler roll adjustment mechanism that is operative to adjustably position the decurler roll to deflect the web from a normal path of travel through the web tension zone and to vary the angle of circumferential wrap of the web on the decurler roll. Preferably, the apparatus also includes a rotatable pivot roll mounted in counterrotating relation with the decurler roll to carry the web in the tension zone in engagement with the inner face of the web. In the preferred embodiment, the pivot roll has a diameter substantially greater than the diameter of the decurler roll and is mounted directly adjacent the decurler roll.
In accordance with the preferred embodiment of the apparatus of the present invention, the decurler roll adjustment mechanism operates to move the decurler roll around the circumference of the pivot roll to simultaneously vary the angle of circumferential wrap of the web on the pivot roll and the decurler roll. More particularly, the adjustment mechanism comprises a pair of mounting brackets, each attached at a radial inner end to a pivot shaft rotatably supporting the pivot roll and at a radial outer end to a journaled connection to one axial end of the decurler roll, and a drive operatively connected to the pivot shaft to rotate the shaft and mounting brackets and to carry the decurler roll in an orbital path around the circumference of the pivot roll.
The decurler apparatus also preferably includes an infeed idler roll mounted upstream of the brake roll and an outfeed idler roll mounted downstream of the pull roll. The apparatus may also include an intermediate idler roll carrying the web to the pull roll. In order to handle web materials which may be delivered with either an up curl or a down curl, the apparatus preferably includes a second decurler roll and a second pivot roll positioned downstream of the main decurler roll and pivot roll and upstream of the intermediate idler roll, the second decurler roll and second pivot roll oriented to operate on a web delivered from a second supply roll with its outer face inverted from the web delivered from the main supply roll.
The brake roll includes a brake roll drive that is operative to retard web movement and the pull roll includes a separate pull roll drive that is operative to increase web movement. The brake roll drive and the pull roll drive are independently adjustable to selectively vary web tension in the web tension zone.